Diode-lasers (edge-emitting semiconductor-lasers) provide an efficient source of bright light. Electrical to optical efficiency can be as high as 50%. This high efficiency makes diode-lasers attractive as laser radiation sources for material processing applications and laser welding applications. An individual diode-laser typically has a length between about 1.0 and 1.5 mm. Light is emitted from an aperture that has a height of between about 1.0 micrometers (μm) and 2.0 μm and a width depending on the output power required from the individual diode laser with the width being greater the greater the power required. This width can be as large as 200 μm. The height and width directions of the emitting aperture are usually termed the fast and slow axes, respectively, by practitioners of the art. The quality of a beam emitted from a diode-laser is high in the fast-axis direction but low in the slow-axis direction, with the slow-axis quality being lower the wider the aperture. Beam quality can be quantified in terms of a parameter known as the “etendue” of the beam. The etendue is a product of the cross-section area of a conical beam of light (perpendicular to the propagation direction) and the solid-angle subtended by the light. A high quality beam has a low etendue and a low quality beam has a high etendue. In any axis perpendicular to the direction of propagation it is possible to envisage the etendue as a product of a length and an angle, typically, in units of millimeter-milliradians. A term “Beam Quality Parameter” also measured in mm-milliradians is commonly used in the industry. This is proportional, but not necessarily equal to, the square root of the etendue. In a high-power diode-laser emitter, the etendue in the fast axis direction is low, and the etendue in the slow axis direction can be as great as two orders of magnitude higher. Etendue is an important parameter, because the etendue of an optical system never decreases. A perfect optical system produces an image with exactly the same etendue as a source being imaged.
An individual diode-laser typically does not emit sufficient power for the applications being considered here. When more power is required than one diode-laser can supply, it is usual commercial practice to provide a linear diode-laser array, commonly referred to as a diode-laser bar. In such an array, a plurality of diode-lasers (emitters) are formed on a single substrate (the “bar”). This provides that the emitting apertures of the emitters are aligned in the slow axis direction. The light from all of the emitters must be collected by an optical system of some kind and focused on material being cut or welded. This is a less-than-ideal arrangement, as the etendue of the diode laser bar in the slow-axis direction is the sum of the etendues of the individual emitters. Because of this an optical system for collecting and focusing the beams must deal with a combined beam that is highly asymmetrical. Such a system requires a complicated arrangement of cylindrical and spherical lens elements. Further, the emitters, being on a common substrate, must be connected electrically in parallel. This creates a requirement for a high-current power supply. The cost of such power supplies rises in proportion to the deliverable current.
U.S. Pat. No. 6,044,096 discloses a diode laser bar package in which a multifaceted optical element is used to receive beams from emitters in a diode-laser bar and rotate the transverse axes of the individual beams through 90 degrees such that the beams leave the element aligned one above the other in the fast-axis direction. In such an arrangement, it is the fast-axis etendues that are aligned. In theory, at least, beams from sufficient emitters can be combined in this way such that the sum of the fast-axis etendues of the beams is equal to the slow-axis etendue of an individual beam. This would produce a symmetrical combined beam that could be focused by a conventional lens. The optical element for rotating the beams, however, is exceedingly complex, as two reflective facets must be provided for each emitter. Such an element, if made from a thermally stable material, is not suitable for low cost construction or production in commercial volumes. Further, the arrangement still requires a high-current power supply for the parallel-connected emitters.
There is a continuing need for a diode-laser array capable of providing a combined beam power of at least about 100 W, and preferably 1 kilowatt (kW) or more, and a combined-beam etendue of between about 5 and 50 mm-mrad. Individual emitters in the array must be connectable in series to avoid the requirement for a high-current power supply, even if this requires a high-voltage power supply.